Process for forming a light beam path in a dielectric mirror

ABSTRACT

An optical mirror element includes an optically transmissive element having a first surface and a second surface, and a reflective coating layer on the first surface that defines a mirror surface. A first portion of the first surface does not include the reflective coating layer such that the first portion defines an optically transmissive window in the mirror surface. Q method of forming an optical mirror element having a window portion includes providing an optical element, masking a first portion of a first surface of the optical element, and thereafter applying a reflective coating to the first surface so as to define a reflective surface, wherein the masked portion defines a transmissive region in the reflective surface. The exposed portion of the first surface may be coated with an anti-reflective coating, either before or after the reflective coating is applied.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical devices, and moreparticularly to optical mirror elements having optically transmissivewindows or features.

Devices such as gas analyzers typically include internal cavitiesdefined by two end mirrors. A laser beam or other light source entersthe cavity and reflects back and forth between the mirror end faces toprovide a long path length. A long path length allows for betterabsorption of the light by trace gases, and hence detection of tracegases. Path lengths of between about 1 meter and 100 meters are typicaland path lengths on the order of a kilometer are possible. For aconfocal cavity arrangement, such as may be found in a Herriott Cell,beam entry into the cavity is typically off axis at a certain entrypoint. The beam reflects off of the concave-shaped end mirrors atdiscrete reflection points until it exits the entry point or otherdefined aperture. Typically, the entry point, and other aperture(s), areformed by drilling a hole in the mirror element to allow for entry oflight into the cavity.

For in-the-field applications, such as use of a portable gas analyzer totest trace gases on site, it is desirable to maintain a controlledenvironment within the Herriott Cell cavity. To realize suchapplications, the physical hole(s) are filled with a glass plug to keepthe cavity environment contained and to make the device robust for fielduse (i.e., to prevent contaminants from entering the cavity). However,use of a glass plug can be difficult and costly, and it may introducenoise due to reflections around the perimeter of the hole. Additionally,the process of drilling and filling with a glass plug can be costly andtime-consuming, and may limit the cavity sizes that can be used.

Therefore it is desirable to provide methods and devices that overcomethe above and other problems. In particular, it is desirable to providemirror elements, and methods of manufacturing the same, that are simpleand cost-effective.

BRIEF SUMMARY

The present invention provides optical mirror elements having atransmissive window, methods for making optical mirror elements, anddevices incorporating such optical mirror elements. The optical minorelements are particularly well suited for use in Herriott Cellarrangements.

According to one embodiment, an optical minor element is provided thattypically includes an optically transmissive element having a firstsurface and a second surface, and a reflective coating layer on thefirst surface that defines a mirror surface, wherein a first portion ofthe first surface does not include the reflective coating layer suchthat the first portion defines an optically transmissive window in themirror surface.

According to another embodiment, an optical cavity device is provided.The device typically includes a first minor element having an internalsurface and an exterior surface, wherein a first portion of the interiorsurface comprises a reflective coating defining a reflective surface,and wherein a second portion of the internal surface comprises ananti-reflective coating defining an optically transmissive window in thereflective surface, and a second minor element having an internalsurface and an exterior surface. The device also typically includes ahousing structure configured to hold the first and second mirrors suchthat the internal surfaces are facing each other along a common axis.One example of such a device structure is a Herriott Cell arrangement.

According to yet another embodiment, a method of forming an opticalminor element having a window portion is provided. The method typicallyincludes providing an optical element, masking a first portion of afirst surface of the optical element, and thereafter applying areflective coating to the first surface so as to define a reflectivesurface, wherein the masked portion defines a transmissive region in thereflective surface. In certain aspects, the exposed portion of the firstsurface is coated with an anti-reflective coating, either before orafter the reflective coating is applied.

According to yet another embodiment, a method of forming an opticalminor element having a window portion is provided. The method typicallyincludes providing an optical element, applying a reflective coating toa first surface of the optical element so as to define a reflectivesurface, and removing a first portion of the reflective coating so as toexpose a portion of the first surface of the optical element, whereinthe exposed portion of the first surface defines a transmissive regionin the reflective surface. In certain aspects, the exposed portion ofthe first surface is coated with an anti-reflective coating, eitherbefore or after the reflective coating is applied.

Reference to the remaining portions of the specification, including thedrawings and claims, will realize other features and advantages of thepresent invention. Further features and advantages of the presentinvention, as well as the structure and operation of various embodimentsof the present invention, are described in detail below with respect tothe accompanying drawings. In the drawings, like reference numbersindicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical mirror element having a window portionaccording to one embodiment.

FIG. 2 illustrates a method of fabricating a mirror element having awindow according to one embodiment.

FIG. 3 illustrates a method of fabricating a mirror element having awindow according to another embodiment.

FIG. 4 illustrates an example of a mirror element having an elongated(wedge-shaped) window portion. The wedge shape is indicated bycross-hatching; the circular dotted lines represent optional curvatureindicators (e.g., the element may be concave or convex between theindicators).

FIG. 5 illustrates an example of a mirror element having two separatewindow portions.

FIG. 6 illustrates examples of different arrangements of opticalelements defining a cavity.

FIGS. 7 and 8 illustrate two Herriott Cell embodiments where mirrorelements of the present invention are particularly useful.

DETAILED DESCRIPTION

The present invention provides optical mirror elements having atransmissive window, methods for making optical mirror elements, anddevices incorporating such optical mirror elements. The optical mirrorelements are particularly well suited for use in Harriet cellarrangements.

FIG. 1 illustrates an optical mirror element having a window portionaccording to one embodiment. As shown mirror element 10 has a reflectiveminor surface 15 with a window or aperture 20 that allows light to passthrough the mirror surface. Window 20 may take on a circular,rectangular or any other shape as is desired. FIG. 1 a shows a frontalview of the mirror surface and FIG. lb shows a side view of the minorelement. In one embodiment, the reflective minor surface 15 includes areflective coating layer that defines a minor surface. A first portionof the minor surface does not include the reflective coating layer suchthat that portion defines an optically transmissive window 20 in themirror surface. The minor element 10 shown in FIG. 1 has a flat end face22 and a concave face 24 on which the minor surface 15 is defined. Oneskilled in the art will recognize that the minor element may have anyother shape as desired.

FIG. 2. illustrates a method 100 of fabricating a mirror element 10having a window 20 according to one embodiment. In step 110, a firstsurface of an optically transmissive element, e.g., optical flat, lens,etc, is coated with a dielectric, anti-reflective coating (e.g.,thin-film coatings or interference coatings). In one aspect, a singlestack dielectric coat is applied. For example, coating techniques suchas chemical vapor deposition (CVD), sputtering, physical vapordeposition, physical liquid deposition, chemical liquid deposition(e.g., electroplating) and others may be used as are well known. Theoptically transmissive element may be made of fused silica or otheroptically transmissive material. Coating step 110 may include coatingthe optical element with one or more layers of anti-reflective material.Useful anti-reflective coating materials, according to certain aspects,include oxide layers such as silicon dioxide (SiO₂), TiO₂, Al₂O₃ andtantalum oxide (Ta₂O₅), and/or other oxides including metal oxides, withappropriate thicknesses for the wavelength range of the radiation to beused. For dielectric coatings two materials with different index ofrefractions are needed. Another useful anti-reflective coating is asingle coating of Magnesium Fluoride (MgF).

In step 115, a mask is provided to cover the portion of the firstsurface that will define the window portion. The mask may be a tape orother material as is well known, or it may include a tab or otherelement that is positioned to cover the window portion. One useful tapethat works well in vacuum conditions is Kapton® Tape. In step 120, thefirst surface is coated with a dielectric, reflective coating (e.g.,thin-film coatings or interference coatings) to create the mirrorsurface. For example, coating techniques such as CVD, sputtering,physical vapor deposition, physical liquid deposition, chemical liquiddeposition (e.g., electroplating) and others may be used as are wellknown. This coating step may include coating the optical elements withone or more layers of reflective material. Useful reflective coatingmaterials, according to certain aspects, include oxide layers such assilicon dioxide (SiO₂), TiO₂, Al₂O₃ and tantalum oxide (Ta₂O₅), and/orother oxides including metal oxides, with appropriate thicknesses. Fordielectric reflective coatings two materials with different index ofrefractions are needed with appropriate thicknesses for the wavelengthrange of the radiation to be used. Other useful reflective coatingmaterials, according to certain aspects, include metal coatings (i.e,gold, aluminum, silver, etc.). In step 125, the mask is removed toexpose the window portion 20. For example, where the mask is a tab orother physical feature, the tab may simply be removed. Where the maskincludes a chemical material or layer, the mask may be chemicallyremoved using a solvent or other technique as is well known. It shouldbe appreciated that, in step 110, only the first portion that willdefine the window portion of the first surface of the optical elementneed be processed (e.g., the remainder of the first surface may bemasked).

FIG. 3 illustrates another method 200 of fabricating a mirror element 10having a window 20 according to another embodiment. In step 210, thefirst surface is coated with a dielectric, reflective coating to createthe mirror surface. For example, coating techniques such as CVD,electron beam deposition, sputtering and others as described above maybe used as are well known. This coating step may include coating theoptical elements with one or more layers of reflective material. Usefulreflective coating materials, according to certain aspects, includeoxide layers such as silicon dioxide (SiO₂), TiO₂, Al₂O₃ and tantalumoxide (Ta₂O₅), and/or metals or other oxides including metal oxides,with appropriate thicknesses. Thereafter, in step 220, a portion of thecoated first surface is etched to remove the reflective coating andexpose a window portion 20. Etching may include chemical etching, laserablation or other techniques as are well known. In certain aspects, itis desirable that the window portion 20 have an anti-reflective coating.This advantageously helps prevent back reflections and reduce opticalnoise from passing through the first surface and in particular thedefined window portion. Therefore, in certain aspects, an additionalstep of applying a dielectric, anti-reflective coating layer to theexposed window portion 20 is performed. For example, in one aspect, thereflective portion other than the window portion may be masked and ananti-reflective coating applied to the window portion after step 220.The mask is then removed as above. Alternatively, prior to step 210, ananti-reflective coating is applied to the optical element (or a portionthereof) similar to step 110 of FIG. 1. In this case, etching 220 isperformed to expose the anti-reflective coating layer and define theshape of the window(s).

It should be appreciated that one or multiple windows may be formed onany given mirror surface, and the shape of window(s) 20 may be definedby the mask(s) and/or etch parameters used. For example, FIG. 4illustrates an example of a mirror element having an elongated(wedge-shaped) window portion, and FIG. 5 illustrates an example of amirror element having two separate window portions.

It should also be appreciated that the optical element may have anyshape as desired. For example, for Herriott Cell applications it isdesirable that the first surface (mirror surface) of the optical elementhave a concave shape (e.g., has a defined curvature profile). However,it is understood that the first surface may have or include a convexshaped surface, a flat shaped surface or other shaped surface. In oneaspect, the first surface defines an interior surface of a sphere, suchas may be found in an integrating sphere device, for example. Where theoptical element has opposite surfaces (e.g., disk or lens element), thesecond surface of the optical element opposite the first surface mayhave any shape as desired, e.g., concave, flat, convex, etc.Additionally, the optical element may be in the shape of a prism, wherethe first surface (mirror surface) is substantially flat, or have anyother shape as is desired. Further, more than one surface of an opticalelement may be processed according to the present invention to produce amirrored surface (or portion of a surface) having one or more windowelements defined therein.

The coatings applied can of course be tailored to the specificapplication(s) desired. For example, when applying a reflective coating,the reflectivity can be tailored as desired. As one example, a coatingthat provides for greater than about 99.9% reflectivity for wavelengthsbetween 1645 to 1655 nm may be used. Similarly, for anti-reflectivecoatings, the reflectivity may be tailored as desired. As one example,an anti-reflective coating that provides for less than about 0.2%reflectivity for wavelengths between 1645 to 1655 nm may be used. Oneskilled in the art will recognize appropriate materials and processparameters for creating tailored reflective (and anti-reflective)coatings depending on the radiation wavelengths to be used.

FIG. 6 illustrates examples of different arrangements of opticalelements defining a cavity. As shown in FIG. 6 a, a concave-concavearrangement is shown, where the beam enters the cavity through thedefined window 20 and reflects off of discrete reflection points aroundeach mirror element until the beam exits the entry point. FIG. 6 b showsa concave-Plano arrangement and FIG. 6 c shows a concave-convexarrangement.

FIGS. 7 and 8 illustrate two Herriott Cell embodiments where mirrorelements of the present invention are particularly useful. In theHerriott Cell arrangement of FIG. 7, the mirror element includes anentry aperture (e.g., window 20) defined in the center of the mirrorelement, along the axis of the device. The opposite mirror elementrotates around the axis. An alternate fluid cleaning nozzle and brushare used to facilitate cleaning of the opposite mirror element. In FIG.8, the entry aperture (e.g., window 20) is defined toward the peripheryof the lens element as in FIG. 6 a.

It should be appreciated that, as used herein, first surface does notnecessarily mean the first surface that a ray of light encounters wheninteracting with an optical element; rather it refers to the surfacethat is being processed with reflective and/or anti-reflective coatingsto form a mirror surface as discussed herein.

While the invention has been described by way of example and in terms ofthe specific embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art. Embodiments can be usedfor a variety of optical devices including an integrating sphere, aHerriott cell, a White cell, a ring down cavity, an astigmatic Herriottcell, and other devices. Therefore, the scope of the appended claimsshould be accorded the broadest interpretation so as to encompass allsuch modifications and similar arrangements.

1-9. (canceled)
 10. A method of forming an optical mirror element havinga window portion, the method comprising: providing an optical element;masking a first portion of a first surface of the optical element; andthereafter applying a reflective coating to the first surface so as todefine a reflective surface, wherein said masked portion defines atransmissive region in said reflective surface.
 11. The method of claim10, further including applying an anti-reflective coating to at leastsaid first portion of the optical element prior to masking.
 12. Themethod of claim 10, wherein the optical element is an opticallytransmissive lens element.
 13. The method of claim 12, wherein the lenselement includes one of a flat surface, a concave surface and/or aconvex surface.
 14. The method of claim 12, wherein the first surface ofthe optical element is a concave surface.
 15. The method of claim 10,wherein masking includes applying a substance to the optical elementcovering at least said first portion, the method further includingremoving the substance prior to applying the reflective coating.
 16. Themethod of claim 10, wherein masking includes inserting a tab or otherfeature over said first portion prior to applying the reflectivecoating, the tab or other feature having substantially the samedimension as said portion.
 17. The method of claim 10, wherein thereflective coating includes one or more layers of metal and/or oxidematerial.
 18. The method of claim 11, wherein the anti-reflectivecoating includes one or more layers of oxide material and/or a layer ofMgF.
 19. A method of forming an optical mirror element having a windowportion, the method comprising: providing an optical element; applying areflective coating to a first surface of the optical element so as todefine a reflective surface; and removing a first portion of thereflective coating so as to expose a portion of the first surface of theoptical element, wherein said exposed portion of the first surfacedefines a transmissive region in said reflective surface.
 20. The methodof claim 19, wherein removing includes etching the reflective coating.21. (canceled)